Ultrasound can be modulated to generate audible sound in air based on a well-known phenomenon that is referred to as the nonlinear interaction of sound waves or the scattering of sound by sound. The nonlinearity of air provides for a self-demodulation effect. Ultrasound waves can be modulated by an audio signal and radiated from a transducer array into the air as primary waves. The modulated ultrasound waves interact in a nonlinear fashion in air. As a result, they are demodulated and produce the audio signal used to modulate the ultrasound waves.
This principle has been applied to create directed speakers that have a very defined directivity pattern, which it makes it possible to realize an acoustic spotlight. See Yoneyama, M., et al. “The Audio Spotlight: An Application Of Nonlinear Interaction Of Sound Waves To A New Type Of Loudspeaker Design”. J. Acoust. Soc. Am. 73 (5), May 1983. (hereby incorporated by reference in its entirety). Conventional superdirective speakers, e.g., parametric speakers, emit modulated ultrasound waves in a narrow beam so that the demodulated audio can only be heard within the beam. Audio is created at an infinite number of points all along the ultrasonic beam.
Conventional parametric speakers use an ultrasonic transducer to project an ultrasonic carrier signal modulated with an audio signal in a collimated beam. Such speakers typically include a modulator for modulating an ultrasonic carrier signal with an audio signal, a driver amplifier for amplifying the modulated carrier signal, and at least one ultrasonic transducer for projecting the modulated carrier signal through the air as a sound beam. Because of the non-linear propagation characteristics of air, the projected modulated carrier signal is demodulated as it passes through the air, thereby generating the audio signal along the beam path.
Anyone in the path of the collimated ultrasonic beam can hear the demodulated audio. It is desirable to have a directed speaker that can target particular individuals. Attempts to direct audio to specific individuals include installing conventional parametric speakers overhead in the ceiling. However, this solution suffers from various drawbacks, including the impracticality of such installations and high power consumption. Moreover, conventional parametric speakers may not be able to seamlessly deliver audio to a particular individual as the individual is moving.